Advances in Methanol Production and Utilization, with Particular Emphasis Toward Hydrogen Generation Via Membrane Reactor Technology

membranes

Review Advances in Methanol Production and Utilization, with Particular Emphasis toward Hydrogen Generation via Membrane Reactor Technology

Francesco Dalena 1, Alessandro Senatore 1, Marco Basile 2, Sarra Knani 3, Angelo Basile 4 and Adolfo Iulianelli 4,*

1 Chemistry & Chemical Technologies Department, University of Calabria, Cubo 15/D, Via P. Bucci, 87036 Rende, CS, Italy; [email protected] (F.D.); [email protected] (A.S.) 2 Department of Ambient, Territory and Chemical Engineering, University of Calabria, Cubo 44/A, Via P. Bucci, 87036 Rende, CS, Italy; [email protected] 3 Laboratoire de Chimie des Matériaux et Catalyse, Département de Chimie, Faculté des Sciences de Tunis, Université Tunis El Manar, Tunis 2092, Tunisia; [email protected] 4 Institute on Membrane Technology of the Italian National Research Council (CNR-ITM), Via P. Bucci, c/o University of Calabria, Cubo 17/C, 87036 Rende, CS, Italy; [email protected] * Correspondence to: [email protected]

Received: 11 September 2018; Accepted: 14 October 2018; Published: 18 October 2018

Abstract: Methanol is currently considered one of the most useful chemical products and is a promising building block for obtaining more complex chemical compounds, such as acetic acid, methyl tertiary butyl ether, dimethyl ether, methylamine, etc. Methanol is the simplest alcohol, appearing as a colorless liquid and with a distinctive smell, and can be produced by converting CO2 and H2, with the further benefit of significantly reducing CO2 emissions in the atmosphere. Indeed, methanol synthesis currently represents the second largest source of hydrogen consumption after ammonia production. Furthermore, a wide range of literature is focused on methanol utilization as a convenient energy carrier for hydrogen production via steam and autothermal reforming, partial oxidation, methanol decomposition, or methanol–water electrolysis reactions. Last but not least, methanol supply for direct methanol fuel cells is a well-established technology for power production. The aim of this work is to propose an overview on the commonly used feedstocks (natural gas, CO2, or char/biomass) and methanol production processes (from BASF—Badische Anilin und Soda Fabrik, to ICI—Imperial Chemical Industries process), as well as on membrane reactor technology utilization for generating high grade hydrogen from the catalytic conversion of methanol, reviewing the most updated state of the art in this field.

Keywords: methanol; steam reforming; water gas shift; partial oxidation; membrane reactors; hydrogen

1. Introduction In the last century, fossil fuels represented the main source of energy production. These feedstocks are not renewable, are limited and, consequently, are responsible for an instable global market, which leads to a corresponding instability in fuel price. Moreover, fossil fuel exploitation is considered primarily responsible for greenhouse gas (GHG) emissions, contributing to the increase in global warming. Today, the most viable options for the exploitation of fossil fuels for power production result from hydrogen and methanol. The use of hydrogen appears very promising, as it shows the highest energy content per unit of weight (142 kJ/g) over any other known fuel and, furthermore, it is environmentally safe [1]. However, the key issues for wide hydrogen utilization as a new energy

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Membranes 2018, 8, 98 2 of 27 environmentally safe [1]. However, the key issues for wide hydrogen utilization as a new energy carrier are represented by its purification costs and by the difficulties linked to the infrastructure for itscarrier storage are representedand transportation. by its purification costs and by the difficulties linked to the infrastructure for its storageBy contrast, and transportation. methanol is easily stored and transported and can be used as a convenient hydrogen carrier.By It contrast, is also particularly methanol is useful easily storedin the andchemical transported industry and as cana solvent be used and as aas convenient a C1 building hydrogen block forcarrier. producing It is also intermediates particularly useful and in synthetic the chemical hydrocarbons, industry as aincluding solvent and polymers as a C1 building and single-cell block for proteinsproducing [2,3]. intermediates In vehicle andtransportation, synthetic hydrocarbons, methanol can including be mixed polymers with conventional and single-cell petrol, proteins without [2,3]. requiringIn vehicle any transportation, technical modification methanol can to bethe mixed vehicl withe fleet. conventional In fact, most petrol, methanol-fueled without requiring vehicles any currentlytechnical modificationuse M85 fuel, to which the vehicle represents fleet. Ina mixt fact,ure most containing methanol-fueled 85% methanol vehicles and currently 15% unleaded use M85 gasolinefuel, which [4]. represents a mixture containing 85% methanol and 15% unleaded gasoline [4]. Approximately 65%65% of of methanol methanol worldwide worldwide is consumedis consumed for thefor productionthe production of acetic of acid,acetic methyl acid, methyland vinyl and acetates, vinyl acetates, methyl methyl methacrylate methacrylate (MMA), (MMA), methylamines, methylamines, metil-t-butil metil-t-butil etere etere (MTBE), (MTBE), fuel fueladditives, additives, and otherand other chemicals. chemicals. The remainingThe remain portioning portion is converted is converted into formaldehydeinto formaldehyde and other and otherproducts products [3,5,6 ],[3,5,6], as illustrated as illustrated in Figure in Figure1. 1.

5 % of the worldwide methanol conversion methanol worldwide the of % 0

l e e id A E e rs in d c M B u e m hy A T F h a e c M M t il ld ti O th a ce e m A M or F FigureFigure 1. 1. ProductsProducts coming coming from from methanol transformation.

A number of technologies were developed over the years to produce methanol, including several feedstocks, such as natural gas, coal, and biomass or CO —the latter directly recoverable from the feedstocks, such as natural gas, coal, and biomass or CO2—the latter directly recoverable from the atmosphere [3,7]. [3,7]. From an historic point of view, methanol production processes took place before before the the 1660s (by Robert Boyle). An importantimportant contributioncontribution to to its its development development was was by by Paul Paul Sabatier, Sabatier, who who carried carried out out the thehydrogenation hydrogenation of a largeof a large variety variety of functional of functional groups groups by metal-based by metal-based catalysis catalysis [8]. Methanol [8]. Methanol synthesis synthesiswas performed was byperformed BASF (Germany) by BASF in 1923,(Germany) developing in a1923, metal-based developing catalytic a hydrogenationmetal-based catalytic process hydrogenationat high pressure process [9]. The at high BASF pres processsure [9]. has The then BASF been process utilized has since then 1927 been by utilized both DuPont since 1927 and theby bothCommercial DuPont Solvents and the CorporationCommercial inSolvents the USA, Corporat representingion in the the start USA, point representing in the methanol the start production point in theindustry methanol and remainingproduction the industry dominant and technologyremaining the for overdominant 45 years technology [10]. for over 45 years [10]. Successively, in in the 1940s, the SwissSwiss LonzaLonza CompanyCompany producedproduced methanolmethanol industriallyindustrially from electrolytic hydrogen and CO , the latter derived from Ca(NO ) synthesis. The reactant gas electrolytic hydrogen and CO2, the2 latter derived from Ca(NO3)2 synthesis.3 2 The reactant gas purification frompuriﬁcation nitrous from vapors nitrous was vaporsthen developed was then by developed Natta (Italy) by Natta and (Italy)combined and with combined the methanol with the synthesis methanol synthesis from CO and H [11]. from CO and H2 [11]. 2

Membranes 2018, 8, x FOR PEER REVIEW 3 of 26 Membranes 2018, 8, 98 3 of 27 The successive development of the steam methane reforming (SMR) reaction, able to generate syngas (aThe mixture successive of H development2, CO, and CO of2 the) combined steam methane with active reforming Cu/ZnO (SMR) catalysts, reaction, ablemade to it generate possible to operatesyngas at milder (a mixture conditions, of H2, CO, such and COas 3002) combined °C and with100 bar. active This Cu/ZnO was the catalysts, core of made the it“ICI possible process”, proposedto operate in 1966 at milder [3]. conditions, such as 300 ◦C and 100 bar. This was the core of the “ICI process”, proposedIn 1973, induring 1966 [3the]. oil embargo proclaimed by the Organization of Arab Petroleum Exporting CountriesIn in 1973, the duringUSA and the Th oile embargoNetherlands, proclaimed the interest by the toward Organization methanol of Arab exploitation Petroleum as Exporting an alternative automobileCountries fuel in thewas USA particularly and The Netherlands, intense, even the interestthough toward it was methanoldefinitively exploitation banned asafter an alternative the 1990s due automobile fuel was particularly intense, even though it was deﬁnitively banned after the 1990s due to to the harmful methanol combustion products negatively affecting the ozone layer. In the same the harmful methanol combustion products negatively affecting the ozone layer. In the same period, period, methanol market demand grew once again as a consequence of its large utilization as a methanol market demand grew once again as a consequence of its large utilization as a fertilizer fertilizerin agriculture. in agriculture. MethanolMethanol is currently is currently produced produced all all around around thethe worldworld (over (over 90 90 methanol methanol plants plants facilitate facilitate a a productionproduction capacity capacity of ofaround around 110 110 million million tons [[3],3], FigureFigure2): 2): North North and and South South America, America, Europe, Europe, Asia,Asia, and and there there are are also also some some plants plants inin Africa.Africa. Moreover, Moreover, global global methanol methanol consumption consumption is valued is valued at an at an averageaverage of of around 100100 Mt/year, Mt/year, growing growing very ve fastry fast in the in lastthe fourlast four decades decades [7,12]. [7,12].

FigureFigure 2. Methanol 2. Methanol production production expressed expressed in Mt per yearyearof of production production from from 1975 1975 until until 2018 2018 (October). (October).

In theIn thelast lastthree three decades, decades, the the potential potential of ofmethan methanolol utilization utilization as as a ahydrogen hydrogen carrier carrier was also also demonstrated [13]. Furthermore, an extensive range of literature was addressed at hydrogen demonstrated [13]. Furthermore, an extensive range of literature was addressed at hydrogen production through steam reforming reaction [14–20]. Compared to steam methane reforming (SMR) production through steam reforming reaction [14–20]. Compared to steam methane reforming reaction (usually carried out between 800 ◦C and 1000 ◦C), methanol steam reforming (MSR) attracted (SMR)particular reaction interest (usually because carried it takesout between place at 800 signiﬁcantly °C and 1000 lower °C), temperatures, methanol steam around reforming 240–260 ◦ C.(MSR) attractedThe MSR particular process interest can be described because by it threetakes partial place chemicalat significantly reactions: lower temperatures, around 240– 260 °C. The MSR process can be described by three partial chemical reactions: ◦ CH3OH + H2O = CO2 + 3H2 ∆H 298K = +49.7 (kJ/mol), (1) CH3OH + H2O = CO2 + 3H2 ∆H°298K = +49.7 (kJ/mol), (1) ◦ CO + H2O = CO2 + H2 ∆H 298K = −41.2 (kJ/mol), (2) CO + H2O = CO2 + H2 ∆H°298K = −41.2 (kJ/mol), (2) ◦ CH3OH = CO + 2H2 ∆H 298K = +90.7 (kJ/mol). (3) CH3OH = CO + 2H2 ∆H°298K = +90.7 (kJ/mol). (3) The MSR reaction is represented by Equation (1), and Equation (2) is the water gas shift The MSR reaction is represented by Equation (1), and Equation (2) is the water gas shift (WGS) (WGS) reaction, while Equation (3) is the methanol decomposition reaction. Apart from in reaction,Equations while (1) Equation and (3), which(3) is the are methanol endothermic decomposition and involve a reaction. volume increase,Apart from the in WGS Equations reaction (1) is and (3), whichexothermic are andendothermic occurs without and volumeinvolve change. a volume increase, the WGS reaction is exothermic and occurs withoutIn the following, volume change. the state of the art about methanol production, its representative production processes,In the following, the most interestingthe state of advancements the art about on methanol its utilization production, for generating its representative hydrogen, combined production to processes,the application the most of interesting methanol in advancements fuel cells is reported. on its utilization for generating hydrogen, combined to the application of methanol in fuel cells is reported.

2. Methanol Production Methanol is producible from a number of carbon-based feedstocks, such as natural gas, coal, biomass, and CO2. Successively, some of the most important methanol production processes are hence examined in detail and related reactions and technologies discussed in depth.

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2.1. Natural Gas as a Primary Source of Methanol Production Today, around 90% of methanol comes from natural gas [21]. The steps for producing methanol are relatively straightforward and can be summarized to the followings three basic stages: (1) Synthesis gas production; (2) syngas conversion into crude methanol; (3) crude methanol distillation to reach the required purity [22–24]. Both steam and autothermal reforming (SMR and AMR, respectively) of natural gas (Equations (4) and (5)) are needed to generate the syngas mixture (mainly composed by H2, CO, and CO2), even though it can also be obtained via partial oxidation of methane (POM) (Equation (6)) or different carbon-based materials, such as coal, heavy oils, or biogases [25].

CH4 + H2O = CO + 3H2 (4)

CH4 + 2O2 → CO2 + 2H2O (5) 1 CH + O → CO + 2H (6) 4 2 2 2 The composition of syngas normally depends on the S ratio (Equation (7)), represented by the hydrogen and carbon dioxide moles difference over the CO2 and CO moles sum.

moles H − moles CO S = 2 2 (7) moles CO2 + moles CO

In ideal conditions, methanol production should require S = 2. The S number considers the continuous CO2 conversion with hydrogen into methanol via reverse WGS reaction. However, third parameter strictly depends on the adopted feedstock and this is the reason why, for example, if syngas is obtained from natural gas reforming, the S value ranges from 2.8 to 3. The second step to convert syngas into crude methanol is performed in the ranges 50–100 bar and 200–300 ◦C and the process mechanism is resumed below: CO + 2H2 = CH3OH. (8) Reaction (8) is subdivided in two consecutive steps:

CO + H2 = CH2O, (9)

CH2O + H2 = CH3OH. (10) In addition to Reaction (8), methanol is also produced by Reaction (11):

CO2 + 3H2 = CH3OH + H2O. (11)

For the traditional gas phase process, the H2/CO ratio changes from 3:1 to 5:1 and the process requires a WGS stage to boost the hydrogen content. By contrast, for the liquid phase process, its superior heat management capabilities may handle the synthesis gas straight from the generator, with a ratio between 1:1 and 1:2, as generated by coal gasiﬁers [9]. Another issue to be considered is the thermodynamic equilibrium regulating the conversion of synthesis gas, which limits the process to low conversion and, consequently, imposes the recycling of a high amount of unconverted gas. Recycling and cooling duty are the main investment costs of this step process, although the continuous Membranes 2018, 8, 98 5 of 27 research in this ﬁeld has allowed several solid catalysts to be developed, able to maximize methanol yield and selectivity, while lowering the formation of byproducts.

2.1.1. Methanol Production through BASF Process In the second decade of the 20th century, the hydrogenation of carbon monoxide giving methanol over iron-based catalysts at 500 ◦C and high pressure (100 bar) was pursued by Mittash et al. [26]. Successively, they performed methanol production using waste gases derived from ammonia synthesis, observing that this reaction produces various products in addition to methanol (Table1), unfortunately thermodynamically more favored with respect to the latter [11,22]. Another key issue was the low Membranes 2018, 8, x FOR PEER REVIEW 5 of 26 methanol yield caused by pollutants present in the reactant gases, which were responsible for the deactivationfor the deactivation of the Fe-based of the Fe-based catalyst, thecatalyst, reaction the between reaction CO between and Fe CO themselves, and Fe themselves, and the consequent and the penta-coordinateconsequent penta-coordinate complexes formation complexes [11 formation]. [11].

Table 1. Reactions involved in methanol synthesis. Table 1. Reactions involved in methanol synthesis. Long-chain alcohols nCO + 2nH = C H OH + (n − 1)H O Long-chain alcohols nCO + 2nH2 2 = CnnH2n2n+1+1OH + (n − 1)H22O RCH CH OH = RCH CHO + H Aldehydes Ketons RCH2CH22OH = RCH22CHO + H2 2 Aldehydes Ketons 2RCH CHO = RCH COCHRCH + O 2RCH2CHO = RCH22COCHRCH3 3 + adsOads CO ++ 3H3H22 == CHCH44+ + HH22OO Hydrocarbons CO2 + 4H2 = CH4 + 2H2O Hydrocarbons CO2 + 4H2 = CH4 + 2H2O nCO + (2n − 1)H2 = CnH2n+2 + nH2O nCO + (2n − 1)H2 = CnH2n+2 + nH2O Dimethyl ether 2CO + 4H2 = CH3OCH3 + H2O Dimethyl ether 2CO + 4H2 = CH3OCH3 + H2O

Therefore,Therefore, a partpart fromfrom thethe eightheighth groupgroup ofof thethe periodicperiodic system,system, manymany oxide-oxide- andand metal-basedmetal-based catalystscatalysts were usedused forfor thethe hydrogenationhydrogenation reaction,reaction, carrying out this processprocess atat 250–300250–300 barbar andand ◦ temperaturestemperatures betweenbetween 320320 andand 450450 °C.C. At these operatingoperating conditions, the experimental results givengiven byby ZnO/CrZnO/Cr22OO33 andand ZnO/CuO ZnO/CuO catalysts catalysts were were particularly interesting [[9,22,26–29].9,22,26–29]. Based Based on thethe aforementionedaforementioned solid catalysts, two ways were proposedproposed forfor describingdescribing thethe reactionreaction mechanism,mechanism, bothboth involvinginvolving COCO andand HH22 adsorptionadsorption [[30–33].30–33]. Figure 33 illustratesillustrates aa schemescheme ofof the the reaction reaction mechanism mechanism takingtaking placeplace inin fourfour consecutiveconsecutive hydrogenationhydrogenation steps.steps.

FigureFigure 3.3. SchemeScheme ofof thethe ﬁrstfirst reactionreaction mechanismmechanism inin fourfour stages;stages; thethe areasareas inin blueblue colorcolor representrepresent thethe catalystcatalyst surface.surface.

FigureFigure4 4 illustratesillustrates that, in the the second second proposed proposed mechanism, mechanism, both both CO CO and and a hydroxyl a hydroxyl group group are arepresent present on onthe thecatalyst catalyst surface, surface, with with the the CO CO generating generating a a formate formate intermediate; successively, hydrogenationhydrogenation and dehydration reactions are responsibleresponsible for methanolmethanol productionproduction throughthrough aa methoxidemethoxide intermediate.intermediate. These two mechanisms are completelycompletely different, ﬁrstlyfirstly becausebecause theythey dodo notnot formform thethe samesame intermediatesintermediates and,and, secondly,secondly, becausebecause thethe latterlatter isis differentlydifferently bondedbonded toto thethe catalystcatalyst surface.surface. In the mechanism of Figure3 3,, theythey areare bondedbonded withwith CC atoms,atoms, whereaswhereas inin thatthat ofof FigureFigure4 4to to OO atoms.atoms.

Figure 4. Scheme of the second reaction mechanism considering hydrogenation and dehydration reactions.

In recent years, ZnO/Cu-based catalysts were the main subject of several scientific studies [27,28,34– 38]. The mixture between Cu and ZnO creates small aggregates, in which ZnO is aggregated around Cu, forming a core–shell structure responsible for the selectivity of methanol production. This aggregate is formed partially by Cu, part of the shell comprises a Cu/Zn alloy, and the outermost part of the shell is represented by ZnOx (selective to the formation of methanol), where the vacations of oxygen are located, while another part of the aggregate consists of ZnO (Figure 5).

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for the deactivation of the Fe-based catalyst, the reaction between CO and Fe themselves, and the consequent penta-coordinate complexes formation [11].

Table 1. Reactions involved in methanol synthesis.

Long-chain alcohols nCO + 2nH2 = CnH2n+1OH + (n − 1)H2O RCH2CH2OH = RCH2CHO + H2 Aldehydes Ketons 2RCH2CHO = RCH2COCHRCH3 + Oads CO + 3H2 = CH4 + H2O Hydrocarbons CO2 + 4H2 = CH4 + 2H2O nCO + (2n − 1)H2 = CnH2n+2 + nH2O Dimethyl ether 2CO + 4H2 = CH3OCH3 + H2O

Therefore, a part from the eighth group of the periodic system, many oxide- and metal-based catalysts were used for the hydrogenation reaction, carrying out this process at 250–300 bar and temperatures between 320 and 450 °C. At these operating conditions, the experimental results given by ZnO/Cr2O3 and ZnO/CuO catalysts were particularly interesting [9,22,26–29]. Based on the aforementioned solid catalysts, two ways were proposed for describing the reaction mechanism, both involving CO and H2 adsorption [30–33]. Figure 3 illustrates a scheme of the reaction mechanism taking place in four consecutive hydrogenation steps.

Figure 3. Scheme of the first reaction mechanism in four stages; the areas in blue color represent the catalyst surface.

Figure 4 illustrates that, in the second proposed mechanism, both CO and a hydroxyl group are present on the catalyst surface, with the CO generating a formate intermediate; successively, hydrogenation and dehydration reactions are responsible for methanol production through a methoxide intermediate. These two mechanisms are completely different, firstly because they do not form the same intermediates and, secondly, because the latter is differently bonded to the catalyst

Membranessurface. In2018 the, 8, mechanism 98 of Figure 3, they are bonded with C atoms, whereas in that of Figure6 of4 27to O atoms.

FigureFigure 4. Scheme 4. Schemeof the second of thereaction second mechanism reaction cons mechanismidering hydrogenation considering and hydrogenation dehydration reactions. and dehydration reactions. In recent years, ZnO/Cu-based catalysts were the main subject of several scientific studies [27,28,34– 38]. TheIn recentmixture years, between ZnO/Cu-based Cu and ZnO creates catalysts small were aggregates, the main in which subject ZnO ofis aggregated several scientiﬁc around studiesCu, forming[27,28 ,a34 core–shell–38]. The mixturestructure between responsible Cu andfor ZnOthe selectivity creates small of methanol aggregates, production. in which ZnOThis isaggregate aggregated is formed around partially Cu, forming by Cu, a core–shell part of the structure shell comprises responsible a Cu/Zn for the alloy, selectivity and the of outermost methanol production.part of the shell This is aggregate represented is formed by ZnOx partially (selective by Cu, to partthe formation of the shell of comprises methanol), a Cu/Zn where alloy,the vacations and the outermostof oxygen partare located, of the shell while is representedanother part by of ZnOxthe aggregate (selective consists to the formation of ZnO (Figure of methanol), 5). where the Membranesvacations 2018, 8, x ofFOR oxygen PEER are REVIEW located, while another part of the aggregate consists of ZnO (Figure5). 6 of 26

FigureFigure 5. Schematic 5. Schematic representation representation of oxygenoxygen vacancies vacancies in a in Cu/ZnO a Cu/ZnO catalyst. catalyst. Tisseraud et al. [37] hypothesized that the morphostructural arrangement of the molecular Tisseraudaggregate et plays al. [37] a signiﬁcant hypothesized role in the that catalyst’s the morphostructural selectivity towards methanol.arrangement In this of structure, the molecular aggregateboth plays H2 anda significant CO2 are selectively role in the chemisorbed catalyst’s on selectivity different surfaces towards of the methanol. catalyst. InIn fact, this Cu structure, does both H2 and COnot adsorb2 are selectively CO2, but adsorbs chemisorbed hydrogen, albeiton different weakly, while surfaces CO2 isof adsorbed the catalyst. on the surfaceIn fact, of Cu ZnO does not and ZnOx. As already reported in literature [27], H is weakly chemisorbed on metallic Cu due to its adsorb CO2, but adsorbs hydrogen, albeit weakly, while2 CO2 is adsorbed on the surface of ZnO and well-known characteristic to rapidly dissociate and recombine hydrogen. The weakly adsorbed H2 is ZnOx. As already reported in literature [27], H2 is weakly chemisorbed on metallic Cu due to its then poured both on the ZnOx (leading to the formation of methanol) and ZnO part (leading to the well-knownformation characteristic of CO). A schematicto rapidly representation dissociate and of the recombine Cu/ZnO catalytic hydrogen. mechanism The weakly is hence adsorbed shown H2 is then pouredin Figure both6. on the ZnOx (leading to the formation of methanol) and ZnO part (leading to the formation ofTisseraud CO). A etschematic al. [37] also representation reported a Cu-ZnOx/ZnO of the Cu/ZnO catalyst design;catalytic the ZnOxmechanism active phase is hence should shown in Figure 6.completely cover the Cu-sized particles, thus avoiding CO formation (unwanted product), making the catalyst highly selective toward methanol formation.

Figure 6. Schematic representation of the Cu/ZnO catalytic mechanism for methanol synthesis.

Tisseraud et al. [37] also reported a Cu-ZnOx/ZnO catalyst design; the ZnOx active phase should completely cover the Cu-sized particles, thus avoiding CO formation (unwanted product), making the catalyst highly selective toward methanol formation.

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Figure 5. Schematic representation of oxygen vacancies in a Cu/ZnO catalyst.

Tisseraud et al. [37] hypothesized that the morphostructural arrangement of the molecular aggregate plays a significant role in the catalyst’s selectivity towards methanol. In this structure, both H2 and CO2 are selectively chemisorbed on different surfaces of the catalyst. In fact, Cu does not adsorb CO2, but adsorbs hydrogen, albeit weakly, while CO2 is adsorbed on the surface of ZnO and ZnOx. As already reported in literature [27], H2 is weakly chemisorbed on metallic Cu due to its well-known characteristic to rapidly dissociate and recombine hydrogen. The weakly adsorbed H2 is then poured both on the ZnOx (leading to the formation of methanol) and ZnO part (leading to the formationMembranes 2018 of ,CO).8, 98 A schematic representation of the Cu/ZnO catalytic mechanism is hence shown7 of in 27 Figure 6.

Figure 6. Schematic representation of the Cu/ZnOCu/ZnO ca catalytictalytic mechanism mechanism for for methanol methanol synthesis. 2.1.2. Methanol Production through ICI PROCESS Tisseraud et al. [37] also reported a Cu-ZnOx/ZnO catalyst design; the ZnOx active phase shouldThe completely hard experimental cover the conditions Cu-sized imposedparticles, by thus the avoiding BASF process CO formation induced strong (unwanted efforts product), to realize makingmethanol the synthesis catalyst athighly lower selective pressures. toward This wasmethanol achieved formation. by ICI Company (today, Johnson Matthey), which proposed a new methanol synthesis process in the pressure range from 35 to 54 bar and at temperatures from 200 to 300 ◦C. This goal was also due to the development of new Cu-based catalysts

(Cu/ZnO/Al2O3)[28–30], but also to the development of new advanced separation/purification processes of synthesis gas, with the advantage of using sulfur–chlorine-free syngas [31–33]. However, although Cu/Zn-based catalysts proved to be very efficient in the methanol synthesis process, their wide utilization was negatively affected by their limited lifetime and low thermal stability [34]. These issues were overcome by adding alumina to the catalysts, responsible for the enhancement of catalytic stability, while depleting the Cu crystallites’ thermal formation [34–36]. Among several kinds of catalytic systems, Cu/ZnO/Al2O3-based catalysts seem to be the most promising ones. Liao et al. [39] reported significant morphology-dependent Cu–ZnO interactions, while Sun et al. [28] revealed their promotional effects on the catalytic performance of CO2 hydrogenation to methanol. A catalytic cycle is then proposed as a microkinetic model in Figure7, which shows the scheme of a reaction mechanism where methanol may be generated by CO hydrogenation by means of HCOO, HCOOH, CH2COOH, CH2O, CH3O, or HCO, CH2O, CH3O intermediates [39]. It is worth of noting that, among various methanol production processes, differences in methanol yield (expressed in t/d) exist, depending on the feedstocks used; see Table2. Membranes 2018, 8, x FOR PEER REVIEW 7 of 26

2.1.2. Methanol Production through ICI PROCESS The hard experimental conditions imposed by the BASF process induced strong efforts to realize methanol synthesis at lower pressures. This was achieved by ICI Company (today, Johnson Matthey), which proposed a new methanol synthesis process in the pressure range from 35 to 54 bar and at temperatures from 200 to 300 °C. This goal was also due to the development of new Cu-based catalysts (Cu/ZnO/Al2O3) [28–30], but also to the development of new advanced separation/purification processes of synthesis gas, with the advantage of using sulfur–chlorine-free syngas [31–33]. However, although Cu/Zn-based catalysts proved to be very efficient in the methanol synthesis process, their wide utilization was negatively affected by their limited lifetime and low thermal stability [34]. These issues were overcome by adding alumina to the catalysts, responsible for the enhancement of catalytic stability, while depleting the Cu crystallites’ thermal formation [34–36]. Among several kinds of catalytic systems, Cu/ZnO/Al2O3-based catalysts seem to be the most promising ones. Liao et al. [39] reported significant morphology-dependent Cu–ZnO interactions, while Sun et al. [28] revealed their promotional effects on the catalytic performance of CO2 hydrogenation to methanol. A catalytic cycle is then proposed as a microkinetic model in Figure 7, which shows the scheme of a reaction mechanism where methanol may be generated by CO hydrogenation by means of HCOO, HCOOH, Membranes 2018, 8, 98 8 of 27 CH2COOH, CH2O, CH3O, or HCO, CH2O, CH3O intermediates [39].

FigureFigure 7. Scheme 7. Scheme of ofa microkinetic a microkinetic model model of aa reactionreaction mechanism mechanism for for methanol methanol synthesis synthesis over over a a Cu-basedCu-based catalyst. catalyst. With With permission permission of of re reprintprint ofof ElsevierElsevier from from Reference Reference [2]. [2]. Table 2. Methanol yield (t/day) guaranteed by various productive processes, depending on the feedstock. It is worth of noting that, among various methanol production processes, differences in methanol Productive Process Feedstock CH OH Yield (t/Day) yield (expressed in t/d) exist, depending on the feedstocks used;3 see Table 2. BASF Syngas 7.9 × 10−2 Dupont Syngas 1.1·× 10−1 Table 2. Methanol yieldHaldor (t/day) Topsoe guaranteed by various Syngas productive processes,2.4·× 10depe3 nding on the feedstock. ICI Carbonaceous 2.5·× 103 Productive Process Feedstock CH3OH Yield (t/Day) BASF Syngas 7.9 × 10−2 As shown in this table, based on syngas as a primary feedstock the methanol yield varies from Dupont Syngas 1.1·× 10−1 around 7.9 × 10−2 t/d with the BASF process to 2.4 × 103 t/d with the Haldor Topsoe process, Haldor Topsoe Syngas 2.4·× 103 while the ICI process makes it possible to reach 2.5·× 103 t/d using carbonaceous feedstocks. ICI Carbonaceous 2.5·× 103 2.2. Biomass and Char as Feedstocks for Methanol Synthesis The use of biomass and char as feedstocks for methanol synthesis represents a new industrial way to solve the issues of energy demand, waste management, and pollutants emissions [40–42]. In the

case of coal and biomass exploitation to produce methanol, the synthesis process is also similar to the one based on natural gas. In fact, it also foresees three stages, such as syngas production, crude methanol synthesis, and purification. At first, coal or biomass react in a gasifier to be converted into gaseous products, consisting of biogas (mainly CH4 and CO2), syngas (H2, CO2, and CO), hydrogen, and alkaline gases [43]. The gasification process represents a well-known thermochemical technique, useful for converting solid biomass into gaseous mixtures by means of air/oxygen, steam, and flue gases, and it is normally carried out at 800–1000 ◦C[33,44]. Otherwise, an alternative process to conventional gasification is represented by the torrefaction or “mild pyrolysis”, which allows the biomass properties’ optimization, making the combustion of biomass possible at temperatures ranging between 200 and 300 ◦C in anaerobic conditions. Indeed, at this operating temperature, water and other volatile compounds evaporate, leaving the lignocellulosic fraction under degradation with a consequent loss of the initial mass around 20% [31,45]. Membranes 2018, 8, 98 9 of 27

When methanol is synthesized from syngas via conventional gasification of biomass, the latter may be represented by whatever feedstock containing carbon, such as coal or solid wastes. However, conventional biomass gasification processes do not guarantee a syngas stream which is useful for methanol synthesis, due to the formation of tar and char via reduction of carbon oxides. Consequently, the requirements of synthesis gas which is useful for methanol production foresee a small portion of inert gases. The catalysts used for producing methanol from syngas require a CO2:CO:H2 ratio = 5:28:63 [46], while the stage of syngas purification from substances which are environmentally harmful [47,48] is normally realized prior to the entrance of gases in the methanol production process.

2.3. CO2 Hydrogenation for Methanol Generation

The hydrogenation of CO2 to produce methanol (Equation (11)) may represent a viable green strategy for a sustainable development, oriented to produce added-value chemicals while reducing CO2 emissions in the atmosphere [49]. The utilization of CO2 as a feedstock includes several benefits because it is inexpensive, abundant in nature, nontoxic, noncorrosive, and nonflammable and, therefore, safe to handle. Furthermore, it may be easily stored and its transportation may be realized in liquid form under mild pressure. Methanol production from CO2 is hence advantageous not only for the exploitation of non-fossil fuel sources (unlike syngas), but also because it overcomes CO2 sequestration, which is a very costly process, also mitigating the GHG effect through an efficient recycling of CO2. Hence, CO2 could be separated and stored from human or industrial activities as well as by absorption from air, then converted renewably into methanol. The conversion of CO2 into methanol via hydrogenation reaction (Equation (11)) requires a considerable energy supply; consequently, an adequate catalytic system is needed. Considering the actual importance and interest on the CO2 hydrogenation, a recent progress about its challenges and opportunities may be found in Li et al. [50]. This process is conventionally carried out from 35 to 55 bar and 200 ◦C, while the highest active and selective catalysts are based on Cu/ZnO/Al2O3. The whole reaction pathway for the methanol production via hydrogenation reaction is reported below:

CO + 2H2 = CH3OH, (12)

CO2 + H2 = CO + H2O, (13)

CO2 + 3H2 = CH3OH + H2O. (14) From a theoretical point of view, to enhance methanol production in conventional reactors, two main routes (apart from the catalyst improvements) can be also pursued:

(a) Recycling of the unconverted synthesis gas after products separation by condensation; (b) in situ reaction products removal.

However, the catalytic hydrogenation of CO2 using H2 produced with renewable energy is considered a potential path forward for the sustainable production of methanol, but also of lower oleﬁns, higher hydrocarbons, formic acid, and higher alcohols. Furthermore, CO2 hydrogenation represents a promising way to convert it into fuels, among other CO2 hydrogenation paths, needing to evaluate two challenges along with it: (a) Sustainable hydrogen source and (b) dispersed product distribution [51]. Much effort was devoted to solving the former challenge and many scientists have already made great progress in water electrolysis to produce H2 using electricity (Equation (14)) generated with solar or wind or other renewable energy, and water splitting using photocatalytic, photoelectrochemical, or other photochemical processes [52].

2H2O → 2H2 + O2 (15) Membranes 2018, 8, 98 10 of 27

Nevertheless, at the moment, H2 is produced expensively and not renewably in conventional systems, representing a strong issue for the development of this process at a larger scale [37].

3. Methanol Utilization In 2010, global methanol demand achieved 49 million tons and, according to the IHS Markit World Analysis—Methanol 2017, by 2021 the demand will overcome 95 million tons, with China playing the role of leader in this market, with around 54% of world capacity and 46% of global production. It is estimated that, by 2021, nearly one in ﬁve tons of global methanol production will be useful for the methanol-to-oleﬁns process and to satisfy the Chinese chemical demand. In 2000, China represented 12% of global methanol demand, while North America and Western Europe 33% and 22%, respectively. In the previsions of IHS Markit, Northeast Asia (dominated by China) will account for around 70% of global methanol demand, followed by North America with 9% and Western Europe with 8% [53].

3.1. Methanol Transformation into Dimethylether Among other chemicals, methanol utilization for di–methyl–ether (DME) production has received growing interest in the last decade. Indeed, methanol plays an important role as a C1 building block in the petrochemical industry and a consistent fraction of its production is consumed in the manufacturing of DME [54,55], as well as an alternative fuel for automotive applications. DME has an octane number and ignition temperature similar to those of diesel fuel, showing interesting characteristics, such as lower NOX emission and less smoke and engine noise than those of conventional diesel engines; furthermore, it can be handled easily [56]. The current transformation of methanol into DME is performed by a double stage (indirect) synthesis, which is followed by dehydration reaction as represented by Equation (15).

2CH3OH → CH3OCH3 + H2O (16)

A further process involves the direct synthesis of DME from syngas, which proceeds with a syngas having CO:H2 molar ratio equal to 1:1, as described in the reaction reported below [57]:

3CO + 3H2 → CH3OCH3 + CO2. (17)

3.2. Methanol in Fuel Cells Applications: DMFCs A direct methanol fuel cell (DMFC) is classiﬁed as a low temperature polymer electrolyte membrane fuel cell supplied by liquid or vapor methanol [58]. Figure8 shows the schematic diagram of a DMFC, which consists of at least ﬁve main porous layers:

(1) Anode gas diffusion layer (AGDL), (2) anode catalyst layer (ACL), (3) polymer electrolyte membrane, (4) cathode catalyst layer, (5) cathode gas diffusion layer. Membranes 2018, 8, x FOR PEER REVIEW 10 of 26

(1) Anode gas diffusion layer (AGDL), (2) anode catalyst layer (ACL), (3) polymer electrolyte membrane, Membranes(4) cathode2018, 8catalyst, 98 layer, 11 of 27 (5) cathode gas diffusion layer.

FigureFigure 8.8.Schematic Schematic view view of aof direct a direct methanol methanol fuel cellfuel (DMFC). cell (DMFC). With permissionWith permission of reprint of ofreprint Elsevier of fromElsevier Reference from Reference [59]. [59].

TheThe methanolmethanol fedfed diffusesdiffuses throughthrough thethe AGDLAGDL towardstowards ACL.ACL. MethanolMethanol isis oxidizedoxidized toto COCO22 + − (Equation(Equation (15))(15)) atat anode side, allowing allowing six six H H+ toto pass pass through through the the membrane membrane and and six six e− eacrossacross the theexternal external circuit, circuit, generating generating electricity. electricity. At Atthe the cathode cathode side, side, oxygen oxygen from from air air is is reduced toto waterwater throughthrough thethe diffusiondiffusion layerlayer andand catalystcatalyst layer.layer. TheThe membranemembrane actingacting asas anan electrolyteelectrolyte isis coatedcoated withwith aa catalyst,catalyst, enhancingenhancing thethe oxidationoxidation ofof methanolmethanol atat thethe anodeanode sideside andand thethe reductionreduction ofof oxygenoxygen atat thethe cathodecathode side.side. + − Anode: CH3OH + H2O → CO2 + 6H + 6e (18) Anode: CH3OH + H2O → CO2 + 6H+ + 6e− (18) + − Cathode: 1.5O2 + 6H + 6e → 3H2O (19) Cathode: 1.5O2 + 6H+ + 6e− → 3H2O (19) Overall: CH3OH + H2O + 1.5O2 → CO2 + 3H2O (20) Overall: CH3OH + H2O + 1.5O2 → CO2 + 3H2O (20) DMFCs may be subdivided, depending on the way of fuel and oxidant supply as (a) active, DMFCs may be subdivided, depending on the way of fuel and oxidant supply as (a) active, (b) (b) passive and (c) semi-passive. passive and (c) semi-passive. Conventional active DMFCs operate by means of auxiliary devices, such as fuel feed pump, Conventional active DMFCs operate by means of auxiliary devices, such as fuel feed pump, oxidant supplier, CO2 separator, and fuel cell stacks. oxidant supplier, CO2 separator, and fuel cell stacks. By contrast, passive (air-breathing) DMFCs do not operate by auxiliary devices; hence, they need By contrast, passive (air-breathing) DMFCs do not operate by auxiliary devices; hence, they need natural mechanisms, such as convection, capillary, gravity, concentration gradient, or osmosis, natural mechanisms, such as convection, capillary, gravity, concentration gradient, or osmosis, to to deliver fuel and oxidant. Air is supplied by a breathing mechanism to cathode, while methanol is deliver fuel and oxidant. Air is supplied by a breathing mechanism to cathode, while methanol is supplied to anode from a reservoir with a concentration gradient between anode and reservoir [60]. supplied to anode from a reservoir with a concentration gradient between anode and reservoir [60]. Semi-passive DMFCs represents a particular class able to combine active and passive typologies, Semi-passive DMFCs represents a particular class able to combine active and passive typologies, attracting a growing attention for the development of compact portable devices with power below attracting a growing attention for the development of compact portable devices with power below 10 W 10 W [59]. As an example, a number of big companies, such as Motorola, Toshiba, Samsung, and NEC, [59]. As an example, a number of big companies, such as Motorola, Toshiba, Samsung, and NEC, were wereinvolved involved in the in theresearch, research, development, development, and and appl applicationication ofof passive passive DMFCs DMFCs [61]. [61]. Among others,others, Toshiba applied a DMFC as mobile devices, requiring manually filling methanol solution from a cartridge and obtaining oxygen directly from ambient air (palm sized DynarioTM)[62]. Nevertheless, DMFC technology also shows some drawbacks when it comes to its wide diffusion. In particular, even though Nafion utilization has been largely investigated as a polymeric membrane, methanol crossover represents its main negative aspect, being responsible for loss fuel and lower Membranes 2018, 8, 98 12 of 27 overall cell voltage, limiting the application of DMFCs [63]. Therefore, the research in this field is devoted to analyzing valid options to Nafion membranes to reduce the effects of methanol crossover and to allow a wider utilization and commercialization of DMFCs [64].

4. Methanol Exploitation for Hydrogen Generation As affirmed by the International Energy Agency, today the world economy is still depending on the exploitation of fossil fuels, such as oil, coal, natural gas, etc. [65]. Consequently, the increment of the environmental pollution due to CO2 emissions and other dangerous gases originating from burning fossil fuels is driving both industry and academia to research new and green technologies, and renewable feedstocks exploitation as well. Hydrogen is currently seen as a clean energy source, playing a relevant role in refining, chemical, and electronic industry [66]. However, hydrogen presents some drawbacks in terms of the difficulties in its storing and transportation, negatively affecting its wider utilization. Consequently, hydrogen generation from an easily transported liquid source may represent a valid option. Methanol is considered an excellent candidate as a hydrogen carrier, showing low toxicity and easy handling. In the following, different industrial processes involving methanol for hydrogen generation are reported and discussed.

4.1. Methanol Decomposition Reaction According to Equation (21), a methanol decomposition (MD) reaction is an endothermic process able to generate H2 and CO: CH3OH = CO + 2H2 (21) This process is convenient for recovering industrial heat waste at around 200 ◦C[67]. It is particularly important to mention that active catalysts based on palladium and prepared by co-precipitation are able, even at temperatures below 200 ◦C, to catalyze this reaction.

4.2. Methanol–Water Solution Electrolysis Methanol–water solution electrolysis is another process to generate hydrogen using an electrolytic cell. Commonly, the water electrolysis process represents the best way to produce high grade hydrogen quickly [68]. As a consequence, also using a methanol–water solution, hydrogen may be easily produced with a purity varying between 95% and 97%, even though the theoretical voltage of the system is much lower (0.03 V) than in water electrolysis (1.23 V) [69]. During methanol–water solution electrolysis, hydrogen is generated by applying a direct current to the electrolytic cell, as shown in Figure9. At the anode side, methanol and water react, generating CO 2, protons, and electrons. Protons migrate through the polymeric membrane from the anode to the cathode of the electrolytic cell, while electrons migrate to the cathode through the external circuit containing the direct current power supply. Hydrogen generation from methanol–water solution electrolysis is useful for portable power applications, since the start up and shut down are possible very quickly and at relatively low temperature [68]. Membranes 2018, 8, 98 13 of 27 Membranes 2018, 8, x FOR PEER REVIEW 12 of 26

Figure 9.9. Schematic view of methanol–water solution electrolysis. With With permission of reprint of Elsevier from Reference [[70].70].

4.3. Methanol Steam Reforming Reaction for Hydrogen Generation Methanol steam reforming (MSR) has been largely investigated and, currently, the most active catalysts forfor this this process process are are based based on Cu, on which Cu, arewhich cheap are and cheap normally and operatednormally in operated the temperature in the ◦ rangetemperature from 240 range to 260 fromC[ 24071 ].to 260 °C [71]. CH3OH + H2O = CO2 +3H2 (22) CH3OH + H2O = CO2 +3H2 (1) Nevertheless, Cu-based catalysts suffer low stability and a pyrophoric nature and, furthermore, are responsibleNevertheless, for Cu-based a significant catalysts production suffer of low CO. stability Catalyst and deactivation a pyrophoric is normally nature and, due furthermore, to sintering, cokeare responsible deposition, for catalyst a significant poisoning production (chloride, of sulphur),CO. Catalyst and changedeactivation in oxidation is normally state. due The to prevention sintering, ofcoke coke deposition, formation catalyst can be poisoning done using (chloride, an excess sulphu of waterr), and in the change feed and,in oxidatio generally,n state. the The best prevention results are obtainedof coke formation for molar can water/methanol be done using ratioan excess of 1.5:1 of water [72,73 ].in the feed and, generally, the best results are obtainedConsequently, for molar water/methanol the research on ratio MSR of catalysts 1.5:1 [72,73]. which are more active, stable, and responsible for aConsequently, lower CO production the research has beenon MSR greatly catalysts pursued. which In are particular, more active, catalysts stable, able and to responsible be operated for at arounda lower 180CO◦ Cproduction should represent has been a desiredgreatly goalpursued. (indeed, In atparticular, this temperature, catalysts higherable to stability be operated and low at COaround production 180 °C should are expected) represent and, a desired from a goal thermodynamic (indeed, at this point temperature, of view, they higher should stability show and almost low completeCO production conversion. are expected) In the viewpoint and, from ofa thermody fuel cells andnamic fuel point processors of view, integration, they should being show MSR almost an endothermiccomplete conversion. reaction, In the the reformer viewpoint reactor of fuel could cell bes synergisticallyand fuel processors coupled integration, with a high being temperature MSR an protonendothermic exchange reaction, membrane the reformer fuel cell reactor (HT-PEMFC), could be which synergistically works exothermally coupled with in the a high range temperature from 160 to proton exchange membrane fuel cell (HT-PEMFC), which works exothermally in the range from 160 180 ◦C. to 180 °C. Apart from Cu-based ones, other catalysts were used for the MSR reaction, in particular those Apart from Cu-based ones, other catalysts were used for the MSR reaction, in particular those based on palladium, which—compared to nickel, platinum, ruthenium supported on ZnO—showed based on palladium, which—compared to nickel, platinum, ruthenium supported on ZnO—showed lower CO production and higher conversion [74]. Additionally, the utilization of bimetallic catalysts lower CO production and higher conversion [74]. Additionally, the utilization of bimetallic catalysts seems to be an interesting choice to enhance both activity and selectivity. Among others, the best seems to be an interesting choice to enhance both activity and selectivity. Among others, the best activity reported was achieved using Pd/Zn and Pd/Ga bimetallic catalysts and the best selectivity activity reported was achieved using Pd/Zn and Pd/Ga bimetallic catalysts and the best selectivity with Pd/Cd [75]. with Pd/Cd [75]. Regarding the typology of the MSR reactor design, it is worth noting that it could play a direct Regarding the typology of the MSR reactor design, it is worth noting that it could play a direct role in the reaction performance. Therefore, the reactor design has as a target to be as cheap as role in the reaction performance. Therefore, the reactor design has as a target to be as cheap as possible and to maximize both conversion and selectivity, also taking into account that its process possible and to maximize both conversion and selectivity, also taking into account that its process performance is influenced by the flow pattern, velocity profile, pressure drop, and heat transfer [76]. performance is influenced by the flow pattern, velocity profile, pressure drop, and heat transfer [76]. Most of the reactor designs deal with rectilinear channels, pinhole, coil-based, and radial, even though Most of the reactor designs deal with rectilinear channels, pinhole, coil-based, and radial, even conventional reactors (CRs) are currently tubular due to the complexity and higher costs of realizing though conventional reactors (CRs) are currently tubular due to the complexity and higher costs of the other configurations. Nevertheless, recently, the area of micro fuel processors made it possible realizing the other configurations. Nevertheless, recently, the area of micro fuel processors made it possible and easier to realize further reactor designs and, namely, well-structured flat microreactors,

Membranes 2018, 8, 98 14 of 27 Membranes 2018, 8, x FOR PEER REVIEW 13 of 26 andshowing easier benefits to realize over further the convention reactor designsal ones, and, such namely, as higher well-structured surface-to-volume flat microreactors, ratio, smaller showing mean benefitsdistance overof the the specific conventional fluid volume ones, suchto the as reac highertor walls, surface-to-volume better heat and ratio, matter smaller transfer mean properties, distance ofand the flow specific patterns fluid that volume fit with to the reactorreaction walls, needs. better A microreactor heat and matter is defined transfer as a properties,device that andcontains flow patternsmicro structured that fit with features, the reaction with needs.a sub-millimeter A microreactor dimension, is defined in aswhich a device chemical that contains reactions micro are structuredperformed features,in a continuous with a sub-millimeter manner [77]. dimension,Compared into whichthe CRs, chemical microreactors reactions present are performed several inbenefits, a continuous such as mannerenhanced [77 heat]. Compared and matter to transfer the CRs, properties, microreactors reduced present mean several distance benefits, of the suchspecific as enhancedfluid volume heat to and the matterreactor transfer walls, higher properties, surface-to-volume reduced mean ratio, distance etc. of the specific fluid volume to the reactorAs a further walls, highersubdivision, surface-to-volume it is possible ratio,to consider etc. microreactors ranging from 0.1 to 10 cm2 in area Asand a mini-reactors further subdivision, between it 10 is possibleand 200 cm to consider2 [78]. The microreactors latter, however, ranging seemfrom to be0.1 more to 10adequate cm2 in areafor packed and mini-reactors bed applications, between better 10 and combining 200 cm2 [the78]. requests The latter, of however,the typical seem fuel tocells be moresize. adequate for packed bed applications, better combining the requests of the typical fuel cells size. 4.4. Process Intensification Strategy Applied to MSR Reactors 4.4. Process Intensification Strategy Applied to MSR Reactors The MSR reaction originates from a stream containing, in addition to hydrogen, other The MSR reaction originates from a stream containing, in addition to hydrogen, other byproducts, byproducts, such as CO, CO2, and small amounts of reactants. Therefore, in the viewpoint of suchcombining as CO, a COfuel2 ,processor and small and amounts PEM fuel of reactants. cells, hydrog Therefore,en needs in high the viewpoint purity prior of to combining entering ain fuel the processorPEMFC. As and for PEM the industrial fuel cells, natural hydrogen gas needs steam high reforming purity to prior produce to entering hydrogen, in the the PEMFC. reformed As forstream the industrialcoming from natural an gasMSR steam reactor reforming should toneed produce furthe hydrogen,r hydrogen the purification/separation reformed stream coming processes, from an MSRnamely reactor water should gas shift need reaction further hydrogenperformed purification/separation in two reactors operating processes, in series namely at waterhigh and gas shiftlow reaction performed in two reactors operating in series at high and low temperature, partial oxidation temperature, partial oxidation reactor to convert CO into CO2, and pressure swing adsorption for reactor to convert CO into CO , and pressure swing adsorption for separating CO from H . As is separating CO2 from H2. As is2 well known, the aforementioned stages of hydrogen2 purification2 wellimpose known, heavy the costs aforementioned and negatively stages affect of hydrogen the overall purification process imposeefficiency. heavy Thus, costs according and negatively to the affectprinciples the overall of Process process Intensification efficiency. Thus, Strategy, according today to theit is principles well recognized of Process that Intensification membrane Strategy, reactor today(MR) technology it is well recognized plays an thatimportant membrane role as reactor a valid (MR) option technology to the CRs, plays because an important it is able role to as combine a valid optionthe reforming to the CRs, reaction because for generating it is able to combinehydrogen the and reforming its purification reaction in for a single generating stage hydrogenwithout needing and its purificationfurther hydrogen in a single separation stage withoutstages (Figure needing 10) further [79,80]. hydrogen separation stages (Figure 10)[79,80].

Figure 10.10. High grade HH22 generation from natural gas steam reforming:reforming: Conventional vs.vs. membrane reactorreactor process.process.

The concept of MR technology was first introduced in the 1950s, although the utilization of new inorganic materials and the development of high-temperature membrane processes have received

Membranes 2018, 8, 98 15 of 27

The concept of MR technology was ﬁrst introduced in the 1950s, although the utilization of new inorganic materials and the development of high-temperature membrane processes have received much attention in the last three decades. In the ﬁeld of MR utilization for hydrogen generation, a general subdivision of MR applications can be summarized as reported below:

(a) Inorganic membrane reactors [79,81]; (b) self-supported and supported Pd-based membrane reactors [80,82]; Membranes 2018, 8, x FOR PEER REVIEW 14 of 26 (c) Zeolite membrane reactors [83]; (d) biomembranemuch attention reactors in the last [84 three]; decades. In the field of MR utilization for hydrogen generation, a general subdivision of MR applications can be summarized as reported below: (e) photocatalytic membrane reactors [85]. (a) Inorganic membrane reactors [79,81]; However,(b) self-supported most of the and open supported literature Pd-bas dedicateded membrane to reactors hydrogen [80,82]; generation via MR technology is devoted(c) to theZeolite investigation membrane reactors of inorganic [83]; membranes application [82,86–89]. Currently, the operating modality(d) of membranesbiomembrane reactors housed [84]; in a MR can be subdivided as reported below: (e) photocatalytic membrane reactors [85]. • ExtractorHowever, modality: most Theof the membrane open literature selectively dedicated to removes hydrogen hydrogen generation fromvia MR the technology reaction is mixture fordevoted permeation. to the investigation of inorganic membranes application [82,86–89]. Currently, the operating • Distributormodality of modality: membranes Thehoused membrane in a MR can allowsbe subdivided the controlledas reported below: addition of hydrogen to the reaction• Extractor mixture. modality: The membrane selectively removes hydrogen from the reaction mixture for • Contactorpermeation. modality: The membrane emphasizes the contact within reactants and catalyst. • Distributor modality: The membrane allows the controlled addition of hydrogen to the reaction In the nextmixture. sub-paragraphs, special attention is dedicated to the characteristics and hydrogen • separation/purificationContactor modality: performance The membrane of palladium-based emphasizes the contact membranes within reactants and their and utilization catalyst. in MRs for producing highIn the grade next sub-paragraphs, hydrogen, the special latter combiningattention is dedicated in an integrated/intensified to the characteristics and whole hydrogen process the chemicalseparation/purification reaction for transforming performance methanol of palladium-based via reforming membranes reaction and into their hydrogen utilization and in MRs the for hydrogen producing high grade hydrogen, the latter combining in an integrated/intensified whole process the separation/purification stage in an unique system. chemical reaction for transforming methanol via reforming reaction into hydrogen and the hydrogen separation/purification stage in an unique system. 4.5. Applications of Pd-Based Membranes in Membrane Reactors Metallic-based4.5. Applications membranesof Pd-Based Membranes are presently in Membrane usually Reactors applied in gas separation and in MR applicationsMetallic-based [79,82]. In particular, membranes palladium are presently and itsusually alloys applied represent in gas the separation dominant and materials in MR because applications [79,82]. In particular, palladium and its alloys represent the dominant materials because they possess high H2 solubility, compared to other metallic materials (Figure 11), which makes them they possess high H2 solubility, compared to other metallic materials (Figure 11), which makes them useful for preparing H2 perm-selective membranes [89]. useful for preparing H2 perm-selective membranes [89].

Figure 11.FigureHydrogen 11. Hydrogen solubility solubility vs. vs. temperature temperature in diffe differentrent metallic metallic materials. materials. With permission With permission of of reprint of Elsevier from Reference [89]. reprint of Elsevier from Reference [89].

Membranes 2018, 8, 98 16 of 27

The hydrogen transport mechanism through dense Pd and/or Pd-alloy is described by the solution–diffusion, taking place in six steps as reported here below:

1. Hydrogen molecules adsorption from the membrane; 2. dissociation of hydrogen molecules on the membrane surface; 3. reversible dissociative chemisorption of atomic hydrogen; 4. reversible dissolution of atomic hydrogen in the metal lattice of the membrane; 5. diffusion into the metal of atomic hydrogen proceeds from the higher hydrogen pressure to the lower hydrogen membrane side; 6. desorption of recombined atomic hydrogen into molecular form.

Hence, hydrogen permeation through a Pd-based membrane may be described by Equation (23):

n n PH2 phps − plps J = (23) H2 δ where JH2 is the hydrogen permeating ﬂux, PH2 the hydrogen permeability, δ the thickness of the palladium/palladium alloy layer, phps and plps are the hydrogen partial pressures on the high pressure (feed) and low pressure (permeate) sides, respectively, while “n” is the pressure exponent. The n-value can vary between 0.5 and 1, depending on the rate-determining step among the aforementioned (a)–(g) stages. If the rate-controlling step is represented by bulk diffusion through the palladium layer (c), then n-value is equal to 0.5 and Sieverts law is followed (usually valid for thick palladium ﬁlms, >5 µm) (Equation (24)) and, consequently, the membrane shows full hydrogen perm-selectivity.

0.5 0.5 PH2 phps − plps J = (24) H2 δ Otherwise, in the case of mass transport to or from the surface (a,g) or dissociative adsorption (b) or associative desorption (e) representing the rate determining stage, n-value becomes 1, indicating that the processes depend linearly on the concentration of molecular hydrogen. Commonly, n = 1 suggests that the permeation through the palladium is very fast (particularly for palladium layer <5 µm). For thick palladium films, deviation from Sieverts law (n-value > 0.5) can be induced by high hydrogen pressure, decrease in the surface reaction rate after absorption of contaminants, concentration polarization and defects due to pinhole formation. Consequently, hydrogen passes through the Pd-based layer not only for the solution–diffusion mechanism, but also via Knudsen or viscous flow mechanisms (Equation (25)) [89]: 1 JTotal = (25) H2 1 + 1 + 1 JSD JK JHP H2 H2 H2 with JTotal representing the total hydrogen permeating through the membrane, JSD the hydrogen H2 H2 permeating flux via solution/diffusion mechanism, JK the hydrogen permeating flux via Knudsen H2 mechanism, and JHP the hydrogen permeating flux via viscous flow/Hagen–Pouiselle mechanism. H2 Therefore, the combination of a hydrogen perm-selective MR and a reforming process (such as MSR reaction) may produce various synergistic advantages. Indeed, the continuous H2 removal from the reaction side shifts the reaction toward further product formation, allowing it to reach higher conversion (the so-called “shift effect”), while recovering a high grade hydrogen stream in the permeate side, as shown in Figure 12. Membranes 2018, 8, x FOR PEER REVIEW 15 of 26

The hydrogen transport mechanism through dense Pd and/or Pd-alloy is described by the solution–diffusion, taking place in six steps as reported here below: 1. Hydrogen molecules adsorption from the membrane; 2. dissociation of hydrogen molecules on the membrane surface; 3. reversible dissociative chemisorption of atomic hydrogen; 4. reversible dissolution of atomic hydrogen in the metal lattice of the membrane; 5. diffusion into the metal of atomic hydrogen proceeds from the higher hydrogen pressure to the lower hydrogen membrane side; 6. desorption of recombined atomic hydrogen into molecular form. Hence, hydrogen permeation through a Pd-based membrane may be described by Equation (21): 𝑃(𝑝 −𝑝) 𝐽 = (20) 𝛿 where JH2 is the hydrogen permeating flux, PH2 the hydrogen permeability, δ the thickness of the palladium/palladium alloy layer, phps and plps are the hydrogen partial pressures on the high pressure (feed) and low pressure (permeate) sides, respectively, while “n” is the pressure exponent. The n-value can vary between 0.5 and 1, depending on the rate-determining step among the aforementioned (a)–(g) stages. If the rate-controlling step is represented by bulk diffusion through the palladium layer (c), then n-value is equal to 0.5 and Sieverts law is followed (usually valid for thick palladium films, >5 μm) (Equation (22)) and, consequently, the membrane shows full hydrogen perm-selectivity. . . 𝑃(𝑝 −𝑝) 𝐽 = (21) 𝛿 Otherwise, in the case of mass transport to or from the surface (a,g) or dissociative adsorption (b) or associative desorption (e) representing the rate determining stage, n-value becomes 1, indicating that the processes depend linearly on the concentration of molecular hydrogen. Commonly, n = 1 suggests that the permeation through the palladium is very fast (particularly for palladium layer <5 μm). For thick palladium films, deviation from Sieverts law (n-value > 0.5) can be induced by high hydrogen pressure, decrease in the surface reaction rate after absorption of contaminants, concentration polarization and defects due to pinhole formation. Consequently, hydrogen passes through the Pd-based layer not only for the solution–diffusion mechanism, but also via Knudsen or viscous flow mechanisms (Equation (23)) [89]: 1 𝐽 = 1 1 1 + + (22) 𝐽 𝐽 𝐽 with 𝐽 representing the total hydrogen permeating through the membrane, 𝐽 the hydrogen permeating flux via solution/diffusion mechanism, 𝐽 the hydrogen permeating flux via Knudsen mechanism, and 𝐽 the hydrogen permeating flux via viscous flow/Hagen–Pouiselle mechanism. Therefore, the combination of a hydrogen perm-selective MR and a reforming process (such as MSR reaction) may produce various synergistic advantages. Indeed, the continuous H2 removal from the reaction side shifts the reaction toward further product formation, allowing it to reach

Membraneshigher conversion2018, 8, 98 (the so-called “shift effect”), while recovering a high grade hydrogen stream17 of 27in the permeate side, as shown in Figure 12.

FigureFigure 12.12. General scheme of a membranemembrane reactorreactor housinghousing aa tubulartubular HH22 perm-selective membrane.

4.6. Methanol Steam Reforming Reaction in Membrane Reactors for Hydrogen Generation In the last two decades, the interest in MSR reaction combined to MRs was reflected in the consistent number of scientific studies present in the open literature, particularly because this combination may offer various benefits for producing hydrogen with respect to the conventional low-pressure systems. Nevertheless, the cost of palladium represents a crucial issue for proposing Pd-based membrane systems as a mature technology, ready for entering in the market. Consequently, taking into account the intrinsic high cost of palladium, the scientific community was oriented to adopt membrane solutions with reduced Pd-content. Table3 shows some of the most representative results in terms of conversion, hydrogen recovery and purity, operating conditions, etc. related to MSR reaction performed in MRs [90–106]. In particular, this table reports the recent advances in composite Pd-based membranes (a dense, thin layer of Pd and/or its alloy deposited on a porous substrate), even considering not Pd-based membranes in order to highlight benefits and drawbacks between the different solutions. In detail, Lytkina et al. [90] prepared two dense unsupported Pd–Ag and Pd–Ru membranes via magnetron sputtering, to be housed in a MR for carrying out an MSR reaction at 300 ◦C and 1:1 feed molar ratio. Reduced methanol conversions were achieved, although reaching a hydrogen recovery of 38% and 18%, respectively, with a purity of around 100%. Saidi [91] performed MSR in a ◦ self-supported Pd–Ag (6 µm thick) MR at 300 C and 2 bar using a Cu/ZnO/Al2O3 catalyst, reaching 98% of conversion, while recovering more than 60% of hydrogen. Liguori et al. [92] used a supported Pd/PSS MR packed with a CuO/ZnO/Al2O3 catalyst; the composite membrane was prepared by electroless plating deposition (ELP), showing an average metallic layer of around 7 µm. At 330 ◦C and 2.5 bar, they reached 85% of methanol conversion and 40% of hydrogen recovery, with a purity of around 100%. Another kind of composite membrane (Pd–Ag/TiO2-Al2O3) was used by Basile et al. [93] for performing an MSR reaction. Nevertheless, the presence of defects in the separative layer was responsible for poor performance, although operating at a relatively higher temperature (550 ◦C): 65 of methanol conversion with a recovered hydrogen stream showing a purity of 72%. Lin et al. [94] used a supported Pd/PSS membrane, prepared by ELP deposition, which showed a metallic layer of 20 µm. In this study, an MSR reaction was performed at 350 ◦C, 6 bar and 1.2:1 feed molar ratio over a Cu-based catalyst, reaching methanol conversion of 95%, 97% of hydrogen recovered in the permeate stream with a purity of 100%. In another work, Lin’s group [97] used a MR allocating a Pd/PSS membrane with the Pd-layer of 20–25 µm. At 350 ◦C, a methanol conversion higher than 99% and pure hydrogen recovered in the permeate stream were reached. Membranes 2018, 8, 98 18 of 27

Table 3. Performance of methanol steam reforming (MSR) reaction in membrane reactors (MRs): Pd/Pd-alloy and not Pd-based membranes application.

Membrane Metallic Conv. H Recovery H Purity Membranes Catalyst H O/CH OH T (◦C) p (bar) 2 2 Ref. Preparation Layer (µm) 2 3 (%) (%) (%) Magnetron Pd-Ag, Pd-Ru 60, 12 Ru/Rh/ZrO 1/1 300 - - 38, 18 ≈100 [90] sputtering 2 Pd-Ag - 6 Cu/ZnO/Al2O3 1/1 300 2 98 64 - [91] Pd/Al2O3 ELP 7 CuO/ZnO/Al2O3 2.5/1 330 2.5 85 >40 ≈100 [92] Pd-Ag/TiO2-Al2O3 ELP - Ru-Al2O3 4.5/1 550 1.3 65 - ≈72 [93] Pd/PSS ELP 20 Cu/ZnO/Al2O3 1.2/1 350 6 ≈95 97 99.9 [94] 3 45 Pd-Ag/α-Al O ELP ~4 CuO/ZnO/Al O 1/1 250 100 ≈100 [95] 2 3 2 3 10 95 Pd-Ag/PSS ELP 20–25 CuO/ZnO/Al2O3 1.2/1 240 10 36.1 18 - [96] Pd/PSS ELP ~20–25 Cu-based 1.2/1 350 - 99 - ≈100 [97] CuO/Al O /ZnO Pd-Ag Cold-rolling 50 2 3 3/1 300 3 - 80 ≈100 [98] MgO Pd-Ru-In - 200 Cu/ZnO/Al2O3 1.2/1 200 7 ≈90 ≈24 ≈100 [99] Pd-Cu - 25 Cu-Zn based - 300 10 >90 ≈38 ≈100 [100] Carbon molecular sieve Pyrolysis - CuO/ZnO/Al2O3 4/1 200 1 ≈95 ≈84 - [101] CuO/Al O /ZnO Carbon supported Pyrolysis - 2 3 3/1 250 2 55 - ≈80 [102,103] MgO Carbon supported - - Cu/ZnO/Al2O3 1.5/1 250 2 ≈99 - 97 [104] SiO2/γ-Al2O3/Pt-SiO2/PSS Soaking-rolling - Cu-Zn based 1.3/1 230 - 100 ~10 - [105] SiO2/γ-Al2O3 Soaking-rolling - Cu-Zn based 3/1 260 - 42 5 98 [106] Membranes 2018, 8, 98 19 of 27

Israni and Harold [95] carried out an MSR in a MR housing a thin Pd–Ag/Al2O3 membrane, having a separative layer of around 4 µm, prepared by ELP deposition. At 250 ◦C and stoichiometric feed molar ratio, they got complete conversion with a hydrogen recovery ranging from 45% at 2.5 bar to 95% at 10 bar, with a hydrogen purity ~100%. Poor results were reached by Rei et al. [96], who used a 20–25 µm thick Pd–Ag/PSS MR. At 240 ◦C and 10 bar; less than 40% of methanol conversion was obtained, with a poor hydrogen recovery (18%). This probably occurred because the supported membrane showed defects in the separative layer, which negatively affected its perm-selectivity characteristics, determining a low shift effect on MSR reaction, with a consequent low performance. Iulianelli et al. [98] performed an MSR reaction in a self-supported Pd–Ag MR at 300 ◦C, 3 bar, and feed molar ratio = 3:1 over a CuO/Al2O3/ZnOMgO catalyst. The membrane showed a thickness of 50 µm and was prepared by the cold-rolling technique, allowing a hydrogen recovery of 80%, having a purity of 100%. Also Itoh et al. [99] housed in a MR a self-supported membrane based on Pd–Ru–In (200 µm thick), reaching, at 200 ◦C and 7 bar, a methanol conversion of 90% and a hydrogen recovery of 24%. The latter result was probably due to the high thickness of the membrane, which was responsible for low hydrogen permeability and, consequently, determined a reduced removal of hydrogen from the reaction to the permeate side. Nevertheless, the recovered hydrogen showed 100% of purity. Wieland et al. [100] housed different self-supported Pd-alloyed membranes (Pd–Ag, Pd–Cu. and Pd–V–Pd) in MRs for performing an MSR reaction. Among them, the best performance was reached using a Pd–Cu membrane (25 µm thick), which proved the most stable; at 300 ◦C and 10 bar, they obtained a methanol conversion which was higher than 90%, recovering around 40% of pure hydrogen. Table3 does not report only interesting data about the application of Pd-based membranes in MRs, but also alternative and cheaper solutions with respect to palladium, namely carbon and silica membranes. In particular, Sà et al. [101] realized a carbon membrane via the pyrolysis technique, starting from dense cellulose cupra–amonia hollow ﬁbres. It showed relatively low H2/N2 perm-selectivity, but housed in a MR for carrying out an MSR reaction at 200 ◦C and 1 bar made it possible to reach a conversion of >90% and a hydrogen recovery of >80%. Briceño et al. [102] prepared a supported carbon membrane on a porous ceramic substrate, constituted of TiO2 coated with ZrO2. Hence, different polymeric solutions were deposited as carbon precursors and successively pyrolyzed. The supported carbon membrane was housed in an MR for an MSR reaction and methanol conversion of >50% was obtained at 250 ◦C and 2 bar, while recovering a hydrogen stream with a purity of ~80% [103]. Zhang et al. [104] allocated in an MR a carbon membrane in a tubular shape with 6-mm of internal diameter and wall thickness of ~20–30 µm, sealed inside a stainless steel tube. During MSR at 250 ◦C and 2 bar, methanol conversion was around 100%, with a recovery of hydrogen showing 97% of purity. Lee et al. [105,106] developed two supported silica-based membranes (SiO2/γ–Al2O3/ Pt–SiO2/PSS and SiO2/γ–Al2O3) membranes for carrying out an MSR reaction in an MR. The ﬁrst membrane (SiO2/γ–Al2O3/Pt-SiO2/PSS) was housed in am MR module packed with a Cu–Zn-based catalyst at 230 ◦C. Complete methanol conversion was reached, although the recovery of hydrogen was very poor (~10%). By contrast, the utilization of the second membrane (SiO2/γ–Al2O3) operated in MR at 260 ◦C and feed molar ration 3:1 showed a lower conversion (~40%) and hydrogen recovery (5%), even though the purity of the recovered hydrogen was quite high (98%).

4.7. Photocatalytic Membrane Reactors: Methanol Production from CO2 Reduction Today, photocatalytic membrane reactors (PMRs) are seen as an interesting option for conventional processes in the field of water and air purification and in organic syntheses as well. MR technology shows certain benefits, such as low energy and chemicals consumption, automatic control, and steady/easy operation, making it an ideal subject in several separation processes. Taking these advantages into account, hybrid processes combining photocatalysis and membrane operations are able to achieve a synergistic effect, minimizing environmental and economic concerns. PMRs are then Membranes 2018, 8, 98 20 of 27 considered a green technology, because they guarantee the safety of the photocatalyst used and operate at mild conditions and in continuous configuration, in which catalyst recovery, reaction, and products separation take place simultaneously, with consequent time- and cost-saving [107,108]. Furthermore, the possibility to combine the photocatalytic reactions with solar light utilization proved particularly attractive, making this process really interesting for future industrial applications. In this scientific field, CO2 transformation into organic molecules like methanol has proven to be very attracting, because it is potentially useful for depleting CO2 emissions from industrial streams, obtaining added-value products, and constituting a great goal for the scientific community. In this regard, CO2 reduction by using TiO2 as a photocatalyst both in liquid and in gas phase was the main subject of various scientific studies [109–111]. Among them, Sellaro et al. [109] performed CO2 reduction into methanol in a PMR operated at mild conditions, in which TiO2 was immobilized in Nafion membranes. The reaction was performed under UV light in liquid phase using H2O as a reducing agent by housing the membranes in a flat sheet membrane module equipped with a quartz window to allow the irradiation. The proposed membrane configuration associated to the continuous flow modality made the removal of methanol from the PMR possible, avoiding its overoxidation. Pathak et al. [110] allocated a porous optically transparent ionomer Nafion membrane embedded with TiO2 nanoparticles in a PMR to carry out the photocatalytic reduction of supercritical CO2. They found that, among methanol, ethanol, and formaldehyde, their composition was determined by the flow rate and by the weight of TiO2 used. More recently, Pomilla et al. [111] proposed CO2 reduction into fuels in a continuous PMR housing a Nafion membrane with embedded exfoliated C3N4. The PMR converted CO2 in various products, such as methanol, ethanol, acetone, etc., with compositions strongly dependent on H2O/CO2 feed molar ratio and residence time. However, the knowledge about the fundamentals of photocatalysis represents a crucial aspect to better understanding all the parameters able to affect the process under investigation. In the near future, the development of new photocatalysts coupled with PMR utilization will constitute a critical task, keeping in mind clearly that a sustainable process may be pursued when a PMR is combined with the sun as a cheap and clean source of light.

5. Recent European Projects Involving Methanol Production, Utilization, and Transformation The European scientific community as well as the European Commission have concretely contributed in recent years to the development of a methanol platform, in which the main subject of the various European Projects is methanol as a source and/or intermediate for its transformation in other added-value products. These European actions have been realized with the financial support of various scientific projects, briefly illustrated in this paragraph. “MefCO2—Methanol fuel from CO2” has been funded by the European Commission within Horizon 2020 (2014–2020) projects and is coordinated by I-Deals/Every Group (Spain). It aims to produce green methanol as an energy vector from captured CO2 and hydrogen produced using renewable energy surplus. The technology is being designed in a modular intermediate scale, with the aim of being able to adapt it to varying plant sizes and gas composition. “WOODSPIRIT” was a big demonstration project developed by the company BioMCN (The Netherlands) for the production of biomethanol, for use as a transportation fuel additive. The consortium got 199 million euros under the first call for proposals of the NER300 funding EU program for innovative low-carbon technologies, demonstrating the production of biomethanol in a large commercial scale (413,000 t/y) using biomass torrefaction and entrained flow gasification as the new core technologies. The aim of this project was to utilize the biomethanol as a petrol additive for partial replacement of mineral fuel. “BEINGENERGY—Integrated low temperature methanol steam reforming and high temperature polymer electrolyte membrane fuel cell” has been a FP7/JTI-CP-FCH—Joint Technology Initiatives—Collaborative Project (FCH), realized in the period between 2012 and 2016 and coordinated Membranes 2018, 8, 98 21 of 27 by University Porto (Portugal). This project was aimed at proposing a power supply comprising a methanol steam reformer and high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) operating at the same temperature. Additionally, it proposed the application of the Pd-based MR technology for single-stage hydrogen production and of an ionic-liquid-supported polymer membrane ◦ selective for CO2 removal from reformate streams operating at up to 200 C. In the same field of application, The European FP7 project “LIQUID POWER” was developed in the period from 2011–2016 and coordinated by Dantherm Power (Denmark), with the aim of developing a new generation of fuel cell systems for backup power markets and for material handling vehicles, as well as for developing new innovative hydrogen supply methods for onsite methanol reforming. “CEOPS—CO2 loop for Energy storage and conversion to Organic chemistry Processes through advanced catalytic Systems” has been an NMP–FP7 European project devoted to a sustainable approach for the production of methanol from CO2, acting as a precursor for fine chemicals products. It was realized between 2013 and 2016 and was coordinated by the CEA Institute (France). The concept of the project was concentrated on the development of two chemical pathways based on: (1) CO2 to methane conversion, realized with advanced catalysts to promote the efficiency of the electro–catalytic process at the point of CO2 emission (cement works); (2) direct conversion of methane to methanol, realized with advanced catalysts to promote the efficiency of the direct pathway instead of using the current pathway consisting of a steam reforming of CH4, which represents 60–70% of cost of production of current methanol, followed by the CO hydrogenation reaction. “NEMESIS2+—New Method for Superior Integrated Hydrogen Generation System 2+” has been an FCH–JU European Project developed from 2012 to 2015 and coordinated by German Aerospace Center—DLR e.V. (Germany). The principal objective of this project was the development of a small-scale hydrogen generator capable of producing 50 m3·h−1 hydrogen (purity: 5.0) from biodiesel and diesel. In detail, methanol was used as a reactant to convert triglyceride into Fatty Acid Methyl Ester, which currently represents biodiesel. “SUPER METHANOL—Reforming of Crude Glycerine in Supercritical Water to Produce Methanol for Re-Use in Biodiesel Plants” was a CP–FP small- or medium-scale focused research project, developed between 2008 and 2011 and led by B.T.G. Biomass Technology Group BV (The Netherlands). The objective was to produce methanol from crude glycerine, reusing it in the biodiesel plant while improving the energy balance, carbon performance, sustainability, and overall economics of biodiesel production. In practice, the consortium was able to perform the reform of glycerine in supercritical water, and to produce a synthesis gas suitable for direct once-through methanol synthesis. “METAPU—Validation of a Renewable Methanol-based Auxiliary Power System for Commercial Vessels” was an FP6–SUSTDEV-3—Global Change and Ecosystems EU project coordinated by Wärtsilä Corporation (Finland). This project investigated, in the period from 2006–2010, the use of methanol and solid oxide fuel cell technology for shipping. In particular, it allowed to successfully propose methanol utilization for fuel onboard ships, whereas international regulations permit only the carriage of methanol as cargo.

6. Conclusions and Future Trends A comprehensive review on methanol production and application was reported in this work, paying particular attention to the exploitation of methanol through MR technology for hydrogen generation. In this work, it was clearly represented how methanol plays a key role as one of the most signiﬁcative and versatile molecules, with interesting potentials as easily transportable fuel and in the chemical industry (as a solvent and as a C1 building block for producing intermediates and synthetic hydrocarbons) or as a convenient energy carrier for hydrogen generation. Indeed, methanol seems to be the most signiﬁcant product of the future, both for green chemistry development and as a hydrogen carrier. In particular, the review highlighted the most important methanol production cycles coming from biomass and/or CO2, the latter case being particularly important in the viewpoint of the minimization of GHGs production and as a solution to global warming. Membranes 2018, 8, 98 22 of 27

The second part of the review was related to the hydrogen generation via reforming reactions involving methanol as well as to the role of methanol in DMFC applications. Furthermore, the principles of the Process Intensiﬁcation Strategy and the consequent application of MR technology were also discussed. Last but not least, a panoramic view on the most signiﬁcative European Project developed in the last ten years about methanol was also added.

Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

List of Acronyms

ACL Anode Catalyst Layer AGDL Anode Gas Diffusion Layer AMR Autothermal Methane Reforming BASF BadischeAnilin und Soda Fabrik CR Conventional Reactor DME DiMethyl Ether DMFC Direct Methanol Fuel Cell ELP Electroless Plating GHG Greenhouse Gases HT High Temperature ICI Imperial Chemical Industries MD Methanol Decomposition MR Membrane Reactor MSR Methanol Steam Reforming MTBE Methyl Tertiary Butyl Ether NCCC National Carbon Capture Center PEM Proton Exchange Membrane PMR Photocatalytic Membrane Reactor POM Partial Oxidation of Methanol SMR Steam Methane Reforming WGS Water-Gas Shift

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